Seismic surveying or seismic exploration, whether on land or at sea, is accomplished by observing a seismic energy signal that propagates through the earth. Propagating seismic energy is partially reflected, refracted, diffracted and otherwise affected by one or more geologic structures within the earth, for example, by interfaces between underground formations having varying acoustic impedances. The affected seismic energy is detected by receivers, or seismic detectors, placed at or near the earth's surface, in a body of water, or down hole in a wellbore. The resulting signals are recorded and processed to generate information relating to the physical properties of subsurface formations. Some seismic exploration or monitoring may be done passively, or in other words, without generating a seismic energy signal explicitly for the purpose of recording the response. In addition to naturally induced microseismic event, microseismic events may be caused by human operations. This may include any circumstance in which human action changes the stress fields within geological structures in the Earth. Some examples include hydraulic fracturing (sometimes referred to as hydrofracturing or “fracking”), perforation shots, string shots, damming a water flow (like a river or stream), heating the ground, cooling the ground, mining, downhole events like drilling, injecting water or other liquid to displace oil or gas, and the discharge of downhole explosives.
Active and passive seismic monitoring are sometimes done over time, or in other words, in four dimensions (4D). In addition to an image of subsurface formations, 4D monitoring can provide information as to how seismic waves interact with those formations over time, or how the subsurface formations and their contents may change over time. For example, as a producing well is depleted, the introduction of water to displace oil or gas may cause a change in the way the seismic waves interact with the subsurface formations. As another example, fractures are formed during hydraulic-fracturing and the progress and quantity of these fractures can be monitored over time. These fractures occur along a fault plane.
The passive monitoring of fault planes can be advantageous in a variety of circumstances. For example, passive seismic monitoring can indicate the origin time, location and magnitude of earthquakes. Passive seismic monitoring for microseismic events can be used to estimate the location and orientation of a fault plane where a smaller fracture has occurred. Determining the location and orientation of a fault plane can provide insight into subsurface formations, including potential traps for oil and gas. A fault may move porous reservoir rock like sandstone or limestone against an impermeable seal like shale or salt, and if the fault does not leak, oil or gas can pool in the reservoir rock. Additionally, the formation and propagation of fractures by the creation of small fault planes can be beneficial when monitoring the progress of hydraulic fracturing. By monitoring the formation of faults in hydraulic fracturing, oil and gas workers may know when sufficient fracturing has been completed or whether more fluid needs to be pumped into the fracturing well.
One way to partially determine a fault plane associated with a seismic or microseismic event is by determining the moment tensor for the seismic or microseismic event. The moment tensor is a second order symmetrical tensor providing a mathematical representation of the forces generated by the seismic or microseismic event. The moment tensor includes nine generalized couples, or nine sets of two vectors. Each vector represents the force along one axis positioned along another axis (for example, one of the nine vector pairs represents the two divergent forces parallel to the x-axis originating at some location along the y-axis, causing a torque about the y-z plane). The moment tensor and the values of its included vectors depends on the strength of the seismic or microseismic event and the orientation of the fault along which the event occurs.
A moment tensor may be decomposed into double-couple and non-double-couple components. The double-couple components represent shear slippage along the fault plane for a seismic event. The non-double couple components represents other motions or forces, including outward motion or volume changes. For example, most large earthquakes are pure shear events along a pre-existing fault with no volume change and so the non-double-couple component is zero. In contrast, when a fracture is first opening a pure tensile event occurs where the double-couple component is zero. After the initial opening, most fractures progress towards a pure shear event, or a pure double-couple event as is seen for large earthquakes, and then revert back until the closure event of the fracture where the double-couple is zero.
For a given fracture, all microseismic events occurring along that fracture will correspond to a single “global” double couple, as all shear slippage will occur in the same direction on the same fault plane. In other words, the double couple component of the moment tensor for each microseismic event will be similar, showing the same pair of nodal planes. Typical passive seismic monitoring has focused on double couple components of the moment tensor.
Determining the moment tensor of a microseismic event is accomplished by inverting the raw data generated by the microseismic event. Except for certain extremes of non-double couple dominated events, the moment tensor includes two possible solution fault planes, referred to as the two nodal planes. These two planes represent the transition between positive first motions, or compressive forces, and negative first motions, or dilatational forces. For pure double couple events, the two nodal planes are orthogonal. For moment tensors with non-double couple components, the two nodal planes are non-orthogonal. Absent additional data collected separately about the subsurface formations in the region, there is no way to distinguish between the two nodal planes for a single microseismic event. The present disclosure provides a solution to determine which of the two possible solution fault planes corresponds to the actual fault plane.